Logical paths

The logical path between two subarea nodes is a virtual route.

Virtual route (VR)

A virtual route is a bidirectional logical connection between two subarea nodes. At least one end of a virtual route must be in a subarea node that activates virtual routes. All hosts can activate virtual routes. NCP Version 4 Release 2 and higher releases can activate virtual routes.

Eight virtual routes numbered 0 to 7 can be defined between two subarea nodes.

One or more virtual routes must be defined for each forward-reverse explicit route pair. A virtual route places a transmission priority on data traffic using the underlying explicit routes.

You can modify the list of virtual routes and their associated transmission priorities using the virtual route selection function of the session management exit routine, or using the virtual route selection exit routine. For information about using these, see z/OS Communications Server: SNA Customization.

Transmission priority (TP)

The transmission priority identifies the priority of message units flowing over an explicit route during a session. The three possible levels of transmission priority are: 0 (lowest), 1, or 2 (highest).

In general, high-priority messages are routed before low-priority messages. Within a specific transmission priority, messages are routed on a first-in-first-out (FIFO) basis.

The eight virtual routes and three levels of transmission priority provide for the possibility of 24 virtual route and transmission priority pairs between two subarea nodes. In other words, each subarea pair can have virtual routes numbered 0 through 7 between them, and each virtual route can have up to three different transmission priorities.

Route extension: A route extension is a logical connection between a subarea node and a peripheral node. A peripheral node uses local addresses for routing and requires boundary function assistance from an adjacent subarea node to communicate with a nonadjacent subarea node.

As shown in Figure 1, a virtual route must be defined for each explicit route pair. For example, between HOSTA and NCPA1, VR0 is associated with ER0 and reverse ER0, and VR1 is associated with ER1 and reverse ER1. Between HOSTA and NCPA2, VR0 is associated with ER0 and reverse ER0, and VR2 is associated with ER1 and reverse ER1. Data flows at a designated transmission priority over a virtual route. In this arrangement, data can flow over any of the virtual routes at transmission priority 0,1, or 2.

Figure 1. Virtual route and explicit route associations and transmission priority